Switching power conversion systems have relied on manual switches or electro-mechanical switches in order to configure an input voltage to either a full wave or voltage doubler configuration. Where an opto-triac has been used it has not been properly synchronized with the control circuit for proper coordination and control of the entire system. It is desirable to have a configuration circuit controlled by a controller circuit. This ensures that the converter will not operate until an adequate line voltage is available. Further, this method reduces component count and is cost effective.
Power conversion is generally accomplished by the switching method known as pulse width modulation (PWM). In the PWM method, voltage and current transition overlap, resulting in significant switching losses in the controlled switching element. The operating frequency of a converter operating by the PWM method is also limited in part by the parasitic elements in the controlled switching element. Resonance switching methods have been proposed to decrease switching losses by allowing current/voltage transition in the controlled switch to occur as much as possible at zero current or zero voltage. However, the resonance switching requires both a resonance inductor (L.sub.r) and a resonance capacitor (C.sub.r). This arrangement can accomplish switching at the desired time in the cycle, but it allows the energy stored in the parasitic capacitance of the switch to go to waste instead of being utilized for energy conversion. For very fast switching of small energy packets, this energy loss becomes significant.
U.S. Pat. No. 4,860,184 by W. A. Tabisz et al. discloses Half-Bridge zero-voltage switched multi-resonant converters. This invention uses the designed high leakage inductance of the power transformer, and resonant capacitors placed in parallel to the rectifier diodes on the secondary side of the power transformer, to constitute a quasi-resonant converter. Such a converter utilizing leakage inductance as its primary resonant inductive element must place most if not all of its resonant capacitance on the secondary side of the transformer in order to fully account for the leakage inductance, which manifests itself as a secondary side element. Hence it is a secondary side resonance but not a full half sinusoidal resonant converter. This type of power conversion which relies mainly on the leakage inductance of the power transformer, uses a conventional transformer construction in which either the secondary conductors are wound on top of the primary conductors, or vice versa. Further, the transformer turn ratio can be increased to enhance the leakage inductance value. This approach results in a significantly high transformer leakage inductance value, that is sufficient to be the resonant inductor.
One disadvantage of not minimizing leakage inductance, but encouraging or increasing it as Tabisz requires in order to achieve resonance, is that while high frequency switching is achieved, only either turn-on or turn-off losses of the switching element are eliminated, but not both. Also, such designed high leakage inductance is difficult to repeatably manufacture.
A transformer having high leakage inductance stores a significant amount of energy in the transformer core; this energy is then released when the switching element is turned off. The energy so released must be snubbed out by a very large, expensive, and power dissipative resistor/capacitance (RC) snubber or damping circuit so that the switching element will not be destroyed after a few switching cycles. High leakage inductance also increases Electro-Magnetic Interference (EMI) because of the menacing resonant oscillation caused by the leakage inductance and inter-winding capacitance of the transformer. Accordingly, an EMI input filter is required in systems such as Tabisz to reduce or eliminate the EMI. When leakage inductance is high, the magnetizing inductance of the power transformer is low compared to a low leakage inductance transformer. High magnetizing current is a waste, hence the current required for conversion in these systems is higher; this causes higher conduction losses in the switching element. Also, the leakage inductance in series with the parallel combination of both the junction capacitance of the rectifying diode and the resonant capacitor will ring during the time the rectifying diode is conducting. The ringing caused by these elements also has to be snubbed out by a power-dissipating RC circuit, or the ripple on the output of the converter will be high.
U.S. Pat. No. 4,959,765 by A. Weinberg discloses a DC/DC converter using quasi-resonance. This DC/DC converter uses an input capacitor, all stray capacitors including those inherent in the switching element, the transformer leakage inductance, the magnetizing inductance of the power transformer, and an output inductor as its resonant elements. This converter as disclosed has all the problems stated in the case of Tabisz et al. Additionally, its low magnetizing inductance will cause a high magnetizing current which is wasteful. This high magnetizing current also requires a long off time in order to completely demagnetize the transformer core before commencing the next switching period. The obvious disadvantage of long off time is in limiting output power, power density, and most importantly the response to rapid load and line changes because any change in line and or load can be responded to only in the next switching period. This type of system is particularly suited for extra low input voltage converters because of the long off time required to reset the transformer core.
U.S. Pat. No. 4,864,479 by Steigerwald et al. discloses a Full-Bridge Lossless switching converter. This invention uses the parasitic capacitance of the switching element and the transformer leakage and magnetizing inductance to exchange energy. This invention, while demonstrating that leakage inductance can be reduced to a low level of 0.1 uH, is not a resonant converter since the resonant tank is not clearly defined. Rather, inherent parasitics are used to shape the switch wave signal in order to achieve substantially low switching power losses.
Presently available power control integrated circuits are not able to modulate and vary frequencies in the mega Hz range, cannot be plugged/unplugged while hot, and are not common to all power conversion systems within a given series without regard to input and output voltage or power output capability. Further, for precise performance of essential housekeeping functions, it is necessary that most housekeeping functions be implemented on a single integrated substrate, unlike the present mixture of discrete and integrated components. Using an integrated substrate will result in uniform manufacturing repeatability and will be cost effective.